There is an increasing trend towards solvent-free coating systems, both on environmental and economic grounds. In order to approach the high level of performance set by current solvent-containing products, many investigations aimed at minimizing the points of weakness of solvent-free coating systems have been performed in recent years. The points of weakness of these systems reside in the difficulty of attaining the requirement of low molecular weight of their components or expressed another way, the points of weakness reside in the difficulty of attaining the requirement of low viscosity, in order to make it possible to dispense without a solvent, and allow further processing of the manufactured article to be performed immediately, in order to dispense with costly storage and inspection of the article.
Criteria which are easy to measure in this context comprise, in particular, the initial loading capacity under stress. This may be described as the initial strength which only permits direct further processing operations to be carried out once it has reached a certain value.
Criteria which are more difficult to assess relate to problems which arise when using compounds of low molecular weight, and which may be summarized under the collective term of migration.
The latter comprises the migration of compounds of low molecular weight from the coating into the environment. Monomeric diisocyanates or possible lower oligomers are regarded as the principal causes of migration.
This migration can lead to the impairment, and in extreme cases even lead to the complete prevention of further processing. Furthermore, very stringent conditions have been prescribed by the legislature, particularly for products which come into contact with foodstuffs. This is because such products, or their decomposition products, capable of migration, can under some circumstances constitute a health hazard.
The manufacture of composite films or foils for the packaging industry is assuming an ever-widening scope, since a composite film or foil with tailored properties can be built by combining different films or foils. Polyurethanes are the preferred adhesives, since they ensure that the films or foils thereof adhere well underneath each other, are odorless and, on account of their transparency, do not result in the visual impairment of the goods to be packaged.
The common two-component systems which are currently used in industry are polyester polyols which are reacted with prepolymers based on diphenylmethane diisocyanate and containing isocyanate groups. Systems such as these provide good initial strengths, but tend to result in increased migration when they are made up incorrectly (e.g. by the use of too much prepolymer).
Systems which provide some progress in this respect have been proposed in EP 150,444 and EP 464,483. However, the prepolymers containing isocyanate groups which are proposed in EP 150,444, or the polyols containing urea groups described in EP 464,483 can only be manufactured with an associated high expenditure. Moreover, products of this composition are often highly viscous; this can lead to difficulties in their subsequent processing under some circumstances. As stated above, there is, therefore, an existing desire in industry for systems which fulfill or approximate the above-mentioned requirements, namely adhesive systems with a high initial strength which are substantially free from migration and yet have as low a viscosity as possible.
The preferred route to attaining these conditions resides in two-component systems which produce on substrates polymeric polyurethane from low molecular weight polyols and polyisocyanates. So as to rapidly achieve the final properties, systems with an overall functionality greater than two are being sought, in order to rapidly obtain a high molecular weight system due to branching.